DESCRIPTION: The long term objective of this project is to gain insight into the multiple roles of nitric oxide (NO) in regulating blood flow to the optic nerve head (ONH), coupling of neuronal activity with O2 consumption, and interaction with hypercapnic vasodilation. Potentially damaging roles of NO and peroxynitrite will also be investigated, using tissue nitrosylation as a marker of damage. Single and double barrel NO and PO2 microsensors will be used in the cat eye to measure local chemical concentrations and their variations with varying metabolic demand and with impaired blood flow. A noninvasive optical method of measuring intravascular PO2 in the ONH by phosphorescence quenching will be incorporated into the existing microscope-based infrared laser system for laser Doppler flowmetry (LDF). This will allow simultaneous NO, PO2 and relative red blood cell flux (blood flow) to be measured. Another method for quantifying basal NO levels using hemoglobin trapping with in vivo microdialysis will be used. In vivo microdialysis techniques will also be used to measure other substrates in the vitreous humor near the ONH, and at other sites near the retina, that are relevant to the role of NO in neurotoxic events. These studies are important for understanding how ONH blood flow is regulated under normal physiological conditions, and what conditions can reverse the normally neuroprotective role of NO to one where damage can occur. It is expected that these studies will provide insight into both normal physiological processes and pathological conditions relevant to glaucoma and neurodegenerative diseases.